1. Field of the Disclosure
The present invention relates generally to image sensors. More specifically, examples of the present invention are related to circuits that readout image data from image sensor pixel cells with low supply voltage.
2. Background
Image sensors have become ubiquitous. They are widely used in digital cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (CMOS) image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
In a conventional CMOS active pixel sensor, image charge is transferred from a photosensitive device (e.g., a photo diode) and is converted to a voltage signal inside the pixel cell on a floating diffusion node. The image charge can be transferred efficiently from photosensitive device to the floating diffusion when the potential at the floating diffusion is higher than the pinning voltage of the photosensitive device (e.g., VPIN). The sum of the floating diffusion voltage swing and pinning voltage typically limits the supply voltage for the active pixel sensor to a minimum of 2.5-3 Volts. However, there is a continuing demand for active pixel sensors with a supply voltage of less than 2.5-3 Volts as the demands for further miniaturization of active pixel sensors increase.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.